text:
Asymmetric thrust
Let's assume we lose left engine on a twin-engine aircraft during a straight and level flight without a side slip. In order to compensate yawing moment to the left from the right engine we deflect the rudder to the right (picture 12.21), however there appears an unbalanced force to the left behind the CG (rudder force). It is said (at least in the book I study about principles of flight) that there are two ways to balance the side force from the rudder: with wings level (picture 12.23) and by banking slightly towards the live engine. At first I would like to consider the first method. It states that yawing to the live engine gives a side slip force on the keel surfaces behind CG opposite to the rudder force. That's fine, however if it acts not through the CG (what is the case in the picture 12.23) there should as well appear a moment from that force which would again yaw the aircraft to the left? Am i missing something?

I find your question a little confusing. First, all moments will act "through" the cog, that is, the cog will be the axis along which the force will try to rotate the plane.

Julious,  Figure 12.21 is the one you should be looking at.  Here the rudder yaw torque balances the asymmetric thrust torque, keeping the plane straight into the direction of flight.
Minimizing drag is very important to maintain airspeed with an engine out.
Figure 12.23 appears to represent a case where too much rudder as applied, creating a side slip.  This is understandably confusing, but common sense will tell you the directional stability of the aircraft will try to bring it back into line with the direction of flight.  That is what "side force from side slip" is.
Remember, directional stability is there whether you have 0, 1, or 2 engines out!  This is a part of the aerodynamic design of your plane so it flies straight, unless you use the ailerons, elevator, or rudder to tell it not to.

It states that yawing to the live engine gives a side slip force on the keel surfaces behind CG opposite to the rudder force. That's fine, however if it acts not through the CG (what is the case in the picture 12.23) there should as well appear a moment from that force which would again yaw the aircraft to the left?

I don't know whether the side force will create a left yawing moment or not. (The picture seems to suggest that it will.) But in any case, if it does create left yawing moment, you can just add more right rudder to counteract it. There's going to be a point where the yawing moment from the rudder cancels out the yawing moment from everything else.

Tanner Swett, but if add more right rudder in order to counteract yawing moment you will add rudder force to the left as well which will no longer counteract side force from sideslip.

@Julious But that'll just result in a different flight path. There's always going to be some combination of aileron and rudder which results in the wings being level and the flight path being straight (unless you hit the limits of one control or the other).


text:
How airlines are calculating crew member's payroll?
What methods are used by the airlines to calculate the payroll of a commercial airline crew member?
Does it always depend on the number of flight hours in the period, the number of duty days, or some other metric?

I think this is offtopic. It depends a lot on airline, and so it is impossible to have a good answer. Quora has many similar answer (from different people point of view). You will not have the definitive answer, but you will get the feeling on differences between airlines (and between periods).

I worked for four different airlines: 2 commuter carriers and 2 747 international carriers. Each had a different pay scheme, ranging from a straight salary regardless of how many hours were flown to pay by the flight hour only to pay by a combination of flight hours and duty time.

@GiacomoCatenazzi I would disagree that it is offtopic. While it definitely does depend on the airline, he's not asking for numbers specific to an airline, he's asking about the methods that are used, which is definitely answerable. It's not like there are an infinite number of methods - there's just a handful.

As mentioned in the comments, this is different per airline. I'll list some of the options used, though there may be airlines out there with their own unique system not included in this list. Note that I use Flight Attendants as examples, as their payroll information and training requirements is a bit more "open source", but these pay methods carry over to anyone working in the aircraft.
Hourly:
This is by far the system of choice for the larger airlines, as they are more easily able to track performance and efficiency statistics (hours worked v hours needed v pay per hour), as well as the fact that it often ends up being cheaper. Flight Attendants have seen remarkable raises in recent years for most companies, and can make anywhere from $25-60/hr, depending on how long they have been working with that particular airline, or in that particular occupation. This provides an incentive for sticking with your current employer.
Static Salary:
A static salary is similar to what you would find in a regular job. You have a minimum number of hours expected of you, and you get paid a flat rate. In the interest of keeping your job, you would be expected to work many more hours than the minimum, as cabin crew understandably work different amounts each month.
Dynamic Salary:
A dynamic salary is a mix of the above two, and is more of a European method. This type of pay expands or shrinks depending on your hours. There is a maximum number of hours you can work, but a relatively low minimum. If you hit the maximum number of hours, or more, you will receive 100% of your salary. If you work half the number of hours, you receive 50% of your salary, and so on. These salaries are typically much higher than a static salary, but it averages out to about the same as a static salary.
Trips for Pay:
TFP is not the easiest way to do it, but gives the airlines a way to pay the lowest wages! You get paid "per trip", as long as the trip hits the minimum distance to count as a trip. If not, you earn a fraction of "per trip" pay. Depending on the airline, a standard trip is typically between 200-300 miles. If the trip is longer, you get paid an additional rate per mile. This system also tracks time that is blocked off for your duties, so you will still get paid if you are waiting for your next aircraft to arrive, but it is a much lower rate.
Extra Pay:
On top of all those methods of pay, nearly all airlines provide some form of additional pay that is added either per trip, or per month. This can be for special skills or training that a flight attendant has received, or for working through holidays or red-eye flights.
That said, I spoke with an old 747 pilot in my family, and he mentioned that holiday pay is not all it's cracked up to be. He was used to them calling in  pilots to work every Christmas Eve, so he ensured when he got out of work on the 23rd, he would take a swig of brandy as soon as possible so that they couldn't legally force him to work extra hours!

+1 for brandy swig.

source: Stack Exchange

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/

sort: votes


text:
What is the feasibility of solar powered drones?
What is the feasibility of solar powered drones? From my understanding, solar panels are becoming more and more efficient, in terms of weight vs energy captured, and in terms of surface area vs energy captured.
What I am interested in is whether solar panels could be placed on different sized drones (small civilian quadcopter and MQ-9 Reaper), and provide enough solar power to keep the aircraft aloft indefinitely (under a theoretical condition of an "endless sunny day").
If this is not possible, are there are realistic changes that can be made to make it possible, while still keeping the core functionality of these two different drones intact?
For the purposes of this question, instead of focusing on a 100% solar powered drone, we can assume that fuel can be carried for takeoff/climb to loitering altitude, at which point is can switch to 100% loiter.

I am aware of the following similar question, but I'm not interested in all the "contributing factors", such as maintenance, fluids, etc - I'm interested in whether efficient solar power can provide enough energy.
https://aviation.stackexchange.com/questions/13673/what-would-prevent-a-solar-powered-aircraft-staying-aloft-forever

Possible, but unwieldy, currently: https://newatlas.com/solar-powered-quadcopter/55993/    --- Also, please alter your question to be clearly about quadcopter drones (if that is what you want to know) because solar crawlers have been real for years.

@bukwyrm I'm interested in both fixed wing drones, and quadcopters. Presumably, fixed wing drones would be easier to implement this on, as they have more surface area, and less energy required to produce lift at loitering altitude.

what are the 'three different drones' you are talking about in the question? civvy quadcopter and MQ-9 and ...? -- Please alter your question to reflect your general interest in flying drones, regardless of type.

@bukwyrm My bad, originally had three but edited third out to reduce scope. Reflected it in wording now. I specifically label the MQ-9 because while there are fixed-wing drones that are specially made to fly with solar power, I'm interested in functional drones that can perform their core functionality/payload.

Without making the MQ9 much, much bigger it isn't possible and won't be. There just isn't enough surface area to generate the required energy. Certainly not possible for quad-copters because of how much energy they use to fly (it is all electric, whereas aircraft has lift from wings)

@RonBeyer If you could elaborate on that using science/numbers in an answer, I would be really interested to see that. If it's truly not possible, I would want to know exactly how big of a gap there is from possible solar power to energy needed to stay aloft.

You may emphase that your question apply to quadcopters and MQ9 as jt is one of the main difference with other questions about solar powered drone here.

Let's take the MQ-9 as an example...
The MQ-9 is powered by a 900 HP Honeywell TPE331-10GD engine. In order to generate 900 HP, it will take 2100 kVA (assuming 3-phase electric AC motor). 2100 kVA is about 1680 kW. That is 1.68 million watts.
Now, the sun delivers (if you could get 100% of the energy sent to Earth) about 4.8kW per square meter (depends on Latitude, which can range from about 6kW to less than 1kW). This would mean you need about 350 square meters of 100% efficient solar panels.
The problem is that the current "world record" for solar efficiency is 46%, let's round up to 50% for easy math sake. So we have to double our solar panel area to 700 square meters.
Now the wingspan of an MQ-9 is 66 feet (about 20 meters), and a length of 36 feet (11 meters). If the MQ-9 was box shaped, it has a surface area of 220 square meters. Lets assume that 20% of that is real "upward facing" surface area so in reality, if you covered the MQ-9 with the most efficient solar panels in the world, you would only generate (220 * .2 * (4.8 * .46)) about 97kW or about 6% of the power actually required to run the aircraft.
This is also assuming that the extra weight of the batteries/wiring/charge controllers don't make the aircraft heavier or require a larger power plant, and assuming that the 3-ph 2100kVA motor is the same weight as the TPE331-10GD.
This gets even worse for quad-copters because they just don't have the surface area, and don't benefit from aerodynamic lift to reduce the amount of power required to fly.
Sources:
USAF MQ-9 Facts
Wikipedia Solar Cell Efficiency

Thanks for the answer, it's really interesting. Seeing as a Reaper doesn't operate at 100% thrust in any normal situation outside of takeoff, I wonder how much energy it takes to stay at cruising altitude... certainly not 6% though! Looks like a complete redesign would have to be in order to accomplish that.

Just for another data point, the Solar Impulse has a wingspan of 236 feet (72m), surface area of 269.5sq-m and generates 66kW peak. It uses 57kW motors (4x13kW) and max Take-off weight of 4400lbs and cruises at 43mph. Reaper can take off at up to 10,000lbs and fly 230mph. The Solar impulse also has to carry almost 1400lbs of batteries for night/reduced lighting flying.

Check out the solar powered drones used to provide cell coverage in Puerto Rico
https://www.npr.org/sections/thetwo-way/2017/11/17/564879549/faa-approves-drone-as-cell-phone-tower-in-the-sky-for-puerto-rico
The solar panels were land based and the drone tethered in place.  Seems a great compromise - keep the heavy stuff on the ground, but get the antenna/electronics up in the air for line of sight & range.
Free flying solar powered drones are apparently quite feasible per this article    https://www.iflscience.com/technology/solar-powered-drone-could-fly-nonstop-five-years/

Titan Aerospace is currently developing a drone named Solara 50 that is being hailed as an “atmospheric satellite” and has quite a lot to offer in terms of gathering scientific data. The drone will fly at an altitude of over 19,000 meters (65,000 feet) where there is little air traffic and above most weather that could impede its travels. Flying at that height will also give it unobstructed access to the sun, which will power the 3,000 solar cells that cover its 50-meter-long (164 feet) wings. The solar cells will also charge lithium ion batteries stored inside the wing so it can fly at night, which means that this drone can fly uninterrupted for up to 5 years all while producing zero emissions.
The body of the drone is durable carbon fiber, and the 5-kilowatt electric motor will allow the aircraft to cruise at about 96 km/h (60 mph). Though the aircraft itself weighs only 160 kg (350 lbs), it will be able to carry 32 kg (70 lbs) worth of payload up into the stratosphere.
Solara 50 is completely self-piloted; able to take off, cruise, and land on its own. It will function much the same way as a satellite, though it will cost much less to launch. From the air, it would be able to track developing storms, migrating wildlife, vegetation patterns, and it would also have data communications capabilities; a feature that has caught the eye of the social media giant, Facebook.

Yes, I have seen that type of technology at drone conferences. While it's very interesting, it doesn't answer my question.

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


text:
What were DJI drones spying on? What data did they capture?
In late 2017, it was claimed that DJI (a very popular Chinese drone company) was working with the Chinese government to spy on the United States via their drones, or their drone controllers, which are owned by many Americans. This claim was made by the Los Angeles Office of the Immigration and Customs Enforcement Bureau, which cited "moderate confidence" in their findings. This led the military to discontinue the use of DJI drones.
My question is - What specific information could be gleaned from these drones? I would presume navigational data, but perhaps video and audio could be recorded? The data that was collected, what benefit would it provide to China?
Regarding the above, my only assumption would be that the Chinese could track military operations that DJI drones were used in, but I'm not sure if this is accurate, or the only potential purpose. I'd be appreciative of any well-sourced answer.
LA-ICE Analysis

Moderation can move it to Skeptics SE

@jean This isn't a "skeptics" thing, nor is it a conspiracy theory. It's a legitimate question I have, as this DHS memo has been referenced in many discussions in my industry (UAVs), in the past year.

@Matt. Completely agree, so I voted to reopen. It’s a technical question about aircraft. If it’s off topic, there’s something wrong with Aviation Stack Exchange, not something wrong with your question.

@Penguin I think that the "what benefit would it provide to China" part is definitely not on topic here (or anywhere on SE, for that matters), so I would consider removing it before reopening

I doubt anything was gleaned per se, but there was recognized a significant security risk here and the military discontinued the use of the produce for fear of integral malware which could give an unfriendly foreign power a readily available means to spy on other nations.
If you have large numbers of these drones in operation all over the United States and other nations of interest equipped with a GPS receiver, and they make use of a controller linked to a smartphone or other device with internet access, you now effectively have an unwitting air force of spy aircraft operating within your target's airspace where the information transmitted from the drone is readily accessible to intelligence agencies within China.  It probably cannot gather sensitive national security information, but these kinds of drones could be used to uncover non-classified government or military activities which could be of use to Chinese data analysts.  We use to do this on a regular basis using spy satellites over Russia and Eastern Europe and thinking several levels ahead, considering how interference with more mundane activities could have a real impact on.
Analysts in China can glean through users until they find ones doing something of interest.  People operating drones near military installations and ranges, etc can be useful and their video can be analyzed e.g. a kid in Las Vegas who lived near Nellis AFB may have his drone targeted when he/she flies it.  If the video captures images of the airbase, analysts can look at day to day differences for things like base maintenance, vehicles in the motorpool, movement of aircraft on the flightline, etc. and the data could be used to predict patterns of behavior, operations protocol, etc. that can be useful to military intelligence.  It's no wonder the US military stopped using DJI products for that reason.

This is nonsense because all the aparatus to get a army of analysts and processing all the data (not to say sending the data by phone) is a lot more expensive than send a couple of agents to play themselves with a drone near a military base (on the same foot any civil can do)

@jean From a data analytics point of view, it would be trivial once you actually have access to the data, especially with the ability of AI and general content filters to only show critical/important drone feeds/data. Similar to how thousands of people across the world are employed solely to push propaganda on social media.

Social media propaganda is a billionaire business niche and feeds on bits of data very easy to auto process and most important uses a well know network infrastructure. It's not easy to send a big amount of data like that without nobody noticing. ISP/telephony/power users can be first one to get the rogue big stream of data

Not necessarily.  It may not require an army of people, just people or robots which can sift through known locations of drones and flight paths that routinely take place over or near areas of interest and these can be scrutinized on a regular basis.  Malware can be imbedded in apps which can work with foreign intelligence organizations and be preset to use the drone's resources to send back data during opportune times and can be quickly filtered using software designed for the role.  It would not require a warehouse of techs watching every drone that flies.


text:
What does it cost to get a hot air balloon to 30,000ft?
I'm interested to know what it would take to get a hot air balloon, with a 600lb load, to 30,000 feet. Is this cheaper than flying a plane with a similar load to the same altitude?
If a special balloon is needed, what is the cost of that balloon compared to the typical recreational hot air balloon? What is the quantity and cost of fuel necessary to make it to that altitude, and hover for a few minutes prior to descending?

How do you intend to breathe at 30,000 ft. (higher than Mt. Everest)?  How will you stay warm at -70F (-60C)?

@abelenky That's not the subject of this question, but presumably with breathing apparatus and a specialized suit. It's not anything ground breaking. People have flown balloons well over 60,000+ feet before.

If you just want to get a view , you can always send a camera up for almost nothing!  https://www.msn.com/en-au/video/watch/everyday-hobby-drone-reaches-record-33000-feet-the-height-most-planes-use-for-cruising/vp-BBKI1S1

@quietflyer Heh, cool, but not my purpose here. I have a followup question in mind though if this problem turns out to be relatively affordable.

4 passengers and equipment are likely to weigh a bit more than 600 lbs.  150 lbs/person including life support?

@DanPichelman The quantity of passengers doesn't matter - I'm more just interested in weight. 600lbs.

"Its not anything ground breaking"?? I can only find 17 manned balloon flights above FL300 in all of history.  Most of them were backed by the Air Force or other well funded government agencies.  All of them required custom, purpose-built equipment.  5 balloonists died during various attempts.

A Hot Air Balloon should be able to reach 30,000 ft just fine as according to Wikipedia the world record altitude for Hot Air Balloons is 69,852 ft.  Besides the necessary survival gear for the occupants (Pressurized oxygen and environment suits) a special propane tank would be needed as according to wikipedia:

However, if the liquid propane in the fuel tanks is too cold (0 °C/32 °F or less) it does not generate sufficient vapor pressure to adequately feed the burner(s). This can be overcome by charging the fuel tanks with inert gas such as nitrogen or by warming them, with electric heat tapes for example, and insulating them against the cold.

Another thing to consider is that you would need special permission from the FAA to go into Class A airspace which goes from 18,000 to 60,000 feet MSL. Usually only IFR rated aircraft can fly in this airspace.  The FAA may require you to fly with a radio and/or transponder if you do get the permission to fly in this airspace.
I do not know how much propane would be required as I don't fly Hot Air Ballons but it would probably depend on the outside air temperature (less fuel in colder air).

[citation needed] Wikipedia fails to mention something.  The problem with propane tanks freezing isn't from outside air temp.  What makes them freeze is the propane itself flashing to vapor.  This is called the "latent heat of vaporization".  Propane tanks right here on the ground will ice up from being drawn down quickly.

This doesn't seem to answer the question about costs specifically, although indeed there are other practical things to think about

Interesting point about the limitations of regular propane/propane tanks as a fuel source.. I'm looking specifically for how expensive it is though, to estimate cost efficiency compared to other forms of flight.

32F is not the limit on propane tanks.  We have people in this area who launch them somewhat routinely in temps of 10F.  The vaporization temperature of propane is -44F. To compensate for vaporization energy, one can carry more tanks, or use tank heaters.  For high altitude flights, gas balloons are more common.

Of course, once you get above FL600, the airspace changes back to class E.


text:
What passenger airplane has the lowest fuel efficiency?
What is the airplane with the lowest fuel efficiency? To set some parameters, let's limit it to the following:

Must be manufactured between 1960 and modern day.
Must be a non-military passenger airplane.
Must use liquid fuel (100LL, Jet-A, etc.)
Can be a commercial aircraft or GA airplane, but must be manned and have passenger seats. No helicopters!
Spacecraft don't count! (You know you were thinking of this)
Must be in the highest standard seat count configuration for the airplane. You cannot take a large jet like an A380, and remove all of the seats save for one!

Fuel efficiency is calculated in pmpg (Passenger miles per gallon), which is the total miles per gallon of the aircraft, divided by the maximum number of seated passengers. This does not include pilot, copilot, or any associated crew as passengers.
Please link back to credible sources, and show how you arrived at your pmpg in terms of mpg and passenger count.

helicopters of course hell yeah. Like I said, if you mean airplane, say airplane.

@user3528438 Good catch - added that disclaimer!

Still, IMO, a bit too broad. The Cessna 172 (GA SEP aircraft) can also have 2-3 passengers. Maybe limit to turbine powered or some minimum weight / passengers count?

You might want to change it to "lowest best fuel efficiency". That'd fix many of the loopholes like helicopters and passenger count.

@Matt you might want to look at Concorde / Tu-144 or a business jet as the worst in terms of miles per gallon per passenger.

@ElectricPilot Concorde is about ~30pmpg, which is pretty bad, but there's got to be some worse aircraft out there. If we were talking strictly about mpg, and not pmpg, then it would be a serious contender. I haven't done the math on a Tu-144 but as an "aircraft of scale" (with lots of seats), it would likely have a similar outcome.

@Matt I've read few articles about early version of the Tu-144 and one of the retired pilots mentioned that it was consuming close to 40 tonnes per hour. Hard to believe, I know, but it looks to be not far from the truth. I would say that early Soviet Jet liners would be on top of the list of what are you looking for. Once I will gather some data, I will try to update my answer. Old piston and turboprop designs were still much more economical.

@ElectricPilot Did the math, Tu-144 turns out to be 9.8pmpg. It's the top contender right now, but I feel that there's likely something even worse!

Whichever one has to sit on the tarmac for hours with its engines turning but not going anywhere.

TU-144 =  9.78 passenger mpg
TU-144 figures found on another forum, from an official flight planning document:
https://www.airliners.net/forum/viewtopic.php?t=1355819#p19483811
--
By comparison the TU-104A = 15.88 passenger mpg
From an old Aviation Week story on the TU-104:
http://aviationweek.com/blog/tupolev-104-harsh-proof-rapid-soviet-progress-1956
Using that article and assuming 0.8 l per kg and 20% reserve, imperial gallons. These figures (especially TU-104A) are rough and would vary by loading and flight profile.

I was immediately thinking of the Concorde but yes the Tu144 would probably be even worse :D

In production: Depending on the configuration its likely to be some variant of the Boeing Business Jet (BBJ) this is the blanket term for the companies line of custom built aircraft. Many of these are large aircraft built to cary, in some cases, one person. Their fuel to passenger ratio is pretty bad. Lets take take a look at the biggest private Boeing BBJ, this 747-8, which burns 9.9 kg/km (35 lb/mi) or 5.1 Gallons Per Mile, lets say there are 30 people on board that puts at 5.88 PMPG assuming the owner has a few friends.
Out of production: The Concorde burned 5,638 gallons (38338.4 LBS) of fuel an hour and served plenty of charter missions with light passenger loads. The Concorde cruised around Mach 2.02 at altitude that computes to ~1155 Kts which puts them right at 4.88 Gallons Per Nautical Mile, lets assume a light charter load of ~30 passengers (eluded to in the above linked podcast) that brings you 6.15 PMPG

I would say a good contender in terms of worst fuel burn per mile / passenger would be a long range, high cruise speed business jet and Concorde with Tu-144.

Saudi Prince Alwaleed bin Talal al-Saud has his own Airbus A380 while probably not carrying a single person, would probably be worse than the BBJ's.

@RonBeyer I had found that article but after some more digging it appears the plane never really came to be.

This doesn't answer the question regarding pmpg.

@Matt: It's ridiculously unlikely you can beat the Concorde without stretching the definitions pretty hard.

@Joshua I wasn't saying that - I was saying that his answer does not answer within the confines of my question. Plus, the Concorde burned 5,638gph, and traveled 1,354mph, which gives you ~0.24mpg, and with it's passenger configuration of 128 passengers, that's ~30pmpg, far from being the "most inefficient" aircraft, particularly when you take into account ~60's era GA airplanes. If you say "served light loads", then literally any plane would be inefficient when it's doing an empty flight. It's a non-answer.

The question asks for aircraft in maximum passenger capacity configuraton -- so for, say, a Cessna 182, that would be four passengers (plus pilot and copilot).

@ZeissIkon Partially correct - pmpg (which is what's used in industry) does not include crew seats, so you would multiple that mpg by two in order to get the pmpg of a Cessna 182, as it only carries two "passengers".

This answer does meet the requirements of the question.

@Matt I always get confused on that -- 152 is 2 seats, 172 is four, 182 must just carry more luggage or fly faster.  Six seats must be the 210 family.

@JuanJimenez No it doesn't - the question specifically asks for pmpg, and that is not referenced anywhere in this question. Nor do his suggestions follow the criteria laid out.

@JuanJimenez sorry for the delay I was out all day and was intending on adding to this I have updated it with numbers.


text:
Redundancy in Integrated modules avionics
What kind of hardware redundancy is used in integrated modules avionics based architecture for EFIS displays running on a single processor?
My question is specific to redundancy of display hardware driving three different displays. I'm not sure if such kind of thing is implemented before, but I'm trying to run three different displays with a single processor.

Do have a question about a specific aircraft or display system? Different systems use different methods to achieve the needed redundancy. As your question is worded is likely to be closed for being unclear or too broad.

I might be able to answer the question as-is, but are you asking about redundancy in the FMC and FGS hardware driving the display, redundancy between displays, or redundancy in an individual display's hardware?  Also, would this be one processor per display?  What are you trying to accomplish, and what was missing from your previous questions?

My question is specific to redundancy of display hardware driving three different displays. I'm not sure if such kind of thing is implemented before, but I'm trying to run three different displays with a single processor.

Why are you trying to run 3 displays from one processor, rather than each display from its own processor?

Your question is originally asking for "hardware" redundancy, of which there is none. However, there is operating system and software redundancy built to alleviate hardware issues.
Typical computer based avionics like the EFIS must be FAA-compliant with ARINC 653. Of course, in an experimental aircraft, you could potentially get non-compliant avionics, but there's not much of a market for them.
ARINC-653 requires that there be an real time operating system layer implemented as a form of redundancy. This is a special "round-robin" form of operating system, which in layman's terms, means that the operating system will only spend up to a fixed amount of time on a process before hopping to the next one, meaning that a single process hanging will not halt the rest of the partitions. You might have a navigation process that dies, but the GPS process can continue to operate as it is in a separate partition. It also has partial requirements that partitions are not dependent on each other, but instead retrieve their data directly from sensors. This gives another layer of redundancy in case a dependency crashes.

Running three displays off one processor is ill-advised, and would never be FAA-compliant, but it is technically possible. You would however have to heavily modify the operating system, and the "Guest OS" (ARINC 653) layer.
I'm happy to go far more indepth into this on StackOverflow if you choose to ask a follow up answer there, but the technical specifications are a bit out of scope for the Aviation StackExchange.

Broadly speaking the redundancy comes in the fact that there are multiple displays that can usually serve all the functions of the EFIS. Most typical electronic instrumentation installations have at least two screens both of which can typically show primary instrumentation (PFD).
Even some of the smallest drop in options these days are capable of serving as both AI/speed/altitude instruments as well as HSI units so if a pair of two is installed you have redundancy for you primary instrument displays.
Larger airliners like the 737 have multiple displays that can be configured in almost any combination in the event of a failure and seem to be driven by two computers.

The NG's have 6 Display Units (DU's), these display the flight
  instruments; navigation, engine and some system displays. They are
  controlled by 2 computers - Display Electronics Units (DEU's).
  Normally DEU 1 controls the Captains and the Upper DU's whilst DEU 2
  controls the F/O's and the lower DU's. The whole system together is
  known as the Common Display System (CDS).
The DU's normally display the PFD's outboard, ND's inboard, engine
  primary display centre (upper) and engine secondary display lower.
  Although they can be switched around into almost any other
  configuration with the DU selector (shown left).


text:
Why is aircraft marshalling still necessary?
I've seen in a documentary that many airports have people guiding the airplanes to the parking spots (marshals). I've also noticed that the key point is to reach a precise spot on which the nose landing gear has to be placed. Couldn't that be solved with a camera installed on the landing gear itself so the pilots can check whether they go along the lines and finally reach the precise spot?

@RussellMcMahon - that is completelly and utterly false. As someone whose job is software development, that's like you said walking carefully outperforms flying in a 747. The only reason is cost.

Think of it this way, (and I'm going to completely fabricate these numbers): Any extra weight requires extra fuel to be carried. Say 10 kg of weight might require 1 kg extra fuel. Across a fleet of aircraft, even a very small increase in weight can have a profound impact on fuel costs. Said another way, a human marshaller is likely more cost effective than installing such a system. At least for now.

camera installed on the landing gear? What about wingtips? and the top of the tail. or 180 degrees to the right, where some jackass is about to cut me off. or there's some lady chasing her kid, or baggage flying around; debris on the runway, etc. (locusts!) Cat and dogs, living together. Mass hysteria!

@user16289 They do have taxi cameras on large aircraft already because its impractical to have a human marshaller accompany the airplane to the runway.

@Davor You misunderstood what I said / I could have been clearer :-). I meant that the cost of implementing protection measures that meet all eventualities in eg the random-threat rich environment of taxiing amongst people, aircraft, unexpected material and protrusions, un or partially authorised people, ... -  is so far much more $ costly than using x% of a person to do the same thing. A well enough programmed, sensor rich system can do the job better in many cases - but so far it's not worth the cost and effort of covering potential hazards compared to using part of a person to handle it.

There are automated systems that provide the same function as a marshal. They are in place at some airports but not all since upgrades cost money and not all airports may see a use for it.
At smaller fields marshals often serve multiple roles aside from just guiding aircraft they may also handle baggage, pump fuel, drive tugs and all the other various ramp jobs. In this case it may be more cost effective to keep a marshal around than instal a costly automated system that only replaces one of their jobs.

These also may fail in low visibility conditions such as heavy fog, requiring human marshallers to go out and replace the automated system.

@MichaelHampton, isn't the automated system installed at about the same place the marshal is standing anyway?

A Marshall will know to adapt so he can see.  Get fog lights, walk closer, get a second person, etc.

@JanHudec It depends on the model. Some use lasers which get scattered by fog. Other models which use visual imaging from CCD cameras do better in low visibility. The Wikipedia article that Dave linked to in this answer has a good summary of the different types.

@MichaelHampton, oh, right, I thought about issues with pilot seeing the system, but not the issues with the system knowing the position of the aircraft, which is, however, the more tricky part.

Not a disagreement per se - just a comment: as your reference says " ... which gives information to a pilot ... " - a person is still 'in the loop' - it's just one who is (even) more highly qualified than a marshall . Most of the systems cited seem to deal mainly with the actual taxiing and little or not with stray bags, children*, vehicles etc. A-VDGS comes closer than most.

You are talking about aircraft marshalling, not sky marshal.
When an aircraft comes a gate, one of the tasks is to park on a designated spot. A marshal will help in that, and so can a camera do that. But there are many other tasks assigned to a marshal, and cameras cannot do those tasks.
This is part of the broader ramp services. The services related to aircraft directly are:

inserting and pulling chocks
indicating when can engines be started and shut off
navigating on ground
informing pilots if they need to change taxi speed

You will always need boots on the ground to make sure no one gets sucked out of theirs. This would be the last job to go after everything else is automated, if even then.

@Mazura - akin to LSO / Paddles / Batsman  (or current more automated but still "a real person") on a carrier. Many do not realise that the guy with the paddles is a qualified pilot - which makes perfect sense on reflection.


text:
What are some of the fastest UAVs? How do the pilots ensure that their controls are being transmitted to actions?
I would clarify that the drones I refer to are UAVs that were piloted using a control station. I know many military projects include such capabilities but I want to know how pilots work on such supersonic UAVs and fly at such high speeds. I read that it was an alternative to the high risks of piloting and training a supersonic aircraft pilot, but I'm curious about such projects and how they fly at such high speeds and how they know when they're in control and when do they know that these crafts are out of control.

There are not any currently public supersonic remotely piloted aircraft

The fastest is probably the QF-16. I have no idea about what engineering makes that happen though.

This is great fodder for the drone.se (https://area51.stackexchange.com/proposals/119178/drone/) site proposal.

This question is a contradiction.  Drones are, by very definition, unpiloted

@CarloFelicione by definition, yes, but in common usage, most people don't differentiate. I work professionally in the UAV/Drone field and they're generally interchangeable, potentially because all drones nowadays also have the ability to be piloted if need be.

It would appear multiple redundancy and a ton of equipment on the ground are at work here.  Great answer, Matt.

@CarloFelicione The OP may consider any unmanned aircraft (no crew onboard).

@CarloFelicione Unmanned aerial vehicles are drones, and can be piloted using a ground station. Drones like Predator are drones with Remote piloting abilities. UAVs would have been a better word but drone was more easily understood in this context I feel.

That is considered an unpiloted aircraft, whether flown my remote control or not.

@CarloFelicione please provide a source for your specific use of terminology. I would appreciate it

The dictionary.  DRONE: N - A PILOTLESS AND REMOTE CONTROLLED AIRCRAFT.  People really don’t understand the origins or nomenclature of the word.

@CarloFelicione which dictionary to be specific. it's a debated word, you can see the internet and people divided on its terminology. I however, do not wish to take this further.

No, it's not.  Please see Webster's New International Dictionary, The Oxford English dictionary, etc..  These aren't obscure references and most "internet" sources are clueless.  "Drone" was co-opted from this in urban lexicon to describe sUAS quadcopters, but the term applies to any remote controlled or pilotless aircraft or missile.  I suggest consulting your definitions from more reputable sources.

The question of the "fastest drone" is not entirely answerable, since whatever answer I give, I'm sure the United States military has a secret drone that goes twice the speed that we have no idea about. I will however put forth the Lockheed D-21 as one of the fastest piloted drones. It tops out at 2,500mph, and was used as a high altitude aerial reconnaissance UAV.

On the other hand, one of the workhorses of the US Air Force is the Global Hawk, a surveillance drone that travels at 400mph.

Regardless of which one you are flying, the speed of communication is the same. Radio transmissions travel close to, or at, the speed of light. That's roughly 670,616,629mph. That's enough to go around the earth more than seven times per second. Due to this, you have near instantaneous feedback from the drones sensors, and near instantaneous control. Yes, things happen faster at 2,500mph, but it is still very controllable.
Beyond the fact that lag is a non-existent problem, take a look at the control stations these guys use.

Due to the fact that they are on the ground which has no meaningful weight limitations, they can equip themselves with a ton of equipment, information, and controls that they would not otherwise have on an aircraft. A few items shown on their screens are sensor feedback for avionics, navigational data, and mission data. With all of these, you really have no human-based excuse for a trained pilot to "lose control" of their aircraft, excluding instances where their transmission may be jammed by a hostile actor. In this situation, the drone has an automatic "Return To Base" function that activates after you have lost control of the drone for a set amount of time.

To say that "lag is a non-existent problem" is extremely optimistic. Speed of light is only one factor here. For digital video, you can't have a delay less than at least one frame at encoding and another at decoding (and yet another at display), and that's already noticeable. In practice it's multiplies of that. Plus you'll need to re-translate the signal via satellites in many cases, and that also adds some lag.

@Zeus So I've actually had the fortunate opportunity to fly some of these badboys. From the information I was given, the delay was well below a single second, and it seemed like even less. I meant "non-existent", in that it does not have any effect on the operation of the aircraft. Such a small delay would only affect operation in very close quarters dogfighting, which requires lightning like reflexes. And I think it's fairly safe to say that there's no interest in dogfighting drones, as the modern age has come to rely more on missiles and engagements from a distance.

Is Lockheed D-21 actually "piloted"? I am under impression that it just flies pre-programmed path on its own.

Well, for 'normal' airplanes a 0.1 s delay is considered significant and can cause pilot-induced oscillations in certain cases. 0.2 s is really bad and makes flying difficult (when precise control is needed). Did you fly it as a pilot-in-the-loop, with full control like a normal airplane, including landings? My understanding of such systems was that the pilot only provides 'guidance' (even if using a traditional stick), while the control system (autopilot) closes the inner loop.


text:
What plane is this, looks a lot like a BAe-146-100 or an Avro 146-RJ100, however with only 2 engines?
I saw it and took a picture. It is flying out of Phoenix Sky Harbor International Airport. It landed at about 5:27 if that matters. It is a green tail livery that extends downward. The wings are above it like on a C172 however the engines are straight under the wings. The engines are fan engines, not props. It looks like the 146-RJ100 with only 2 engines total though.

Thanks for your help!

please add the photos here, do not link a shared drive where they can be removed any time making this question incomplete.

It's a Fairchild Dornier 328JET

It was a bit of a flop insofar as they stuck turbofans on a straight wing turboprop and ended up with a jet that was too slow to mix with the heavies and regionals flying around at 0.75 or 0.8 Mach, so on busy routes they typically were assigned altitudes under 30000 ft to keep them out of the way of the faster traffic, negating some or most of the benefit of the fan.

I had to look up it’s Mach performance. Yes, M0.63 is pretty slow for a jet.

I learned this when I was at a conference once with regional operators and one of them, that operated 328Js along with other types, told me how frustrated they were that ATC was always forcing their planes down to FL260 or 280 because of the overtaking problem, which did a bit of damage to the airplane's business case.  Why even bother with RVSM cert?

The 328 was developed as a turboprop and marketed to do 345 kts when engineering said it will do 330 tops. So they had to increase installed power and it hit the market when fuel prices were dropping. Management felt it would be a good idea to switch to jets. As a turboprop it was excellent …


text:
Aviation theory for fighter pilots
Background:
I'm an 18-year-old Finn and so I'll be starting my military service next summer. I'm currently applying to the Air Force to become a fighter pilot, but I don't have any previous training in aviation. I have studied a bit of theory from the internet but I just want to ask what would be the most relevant information for someone trying to become a fighter pilot.
So the quistion...
What should someone, who's wanting to become a fighter pilot, try to learn and in what order? And what are good resources for learning those things?
Thank you in advance!

I'm know nothing about the Finnish military but what makes you think a random website will give you better training than your future employer?

@Sanchises I don't. I'm just curious and want to learn in advance. And it might give me some edge when trying to get there :D

I haven’t read this one yet, but it looks like something you might like, Aerodynamics for Naval Aviators: https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/media/00-80T-80.pdf

After you read all of the US FAA books on aviation and taken a few lessons, this one is useful. Stick and Rudder: An Explanation of the Art of Flying
by Wolfgang Langewiesche

Thanks @JScarry , that's interesting. But are you saying that's where I should start from?

I would not start with Aerodynamics for Naval Aviators.  It is dry, not comprehensive, and will not really differentiate you from other applicants.  I do use that book for CFI students, however.

@Miksu No. I think you should start with the free FAA flying publications. https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/ Start with Airplane Flying Handbook, then Instrument Flying Handbook. There are also a ton of YouTube videos out there to get a feel for real flying.

My experience is with US students who have joined the US Air Force, Navy or become Army Aviators.
My non-flight study guide includes:

Critical thinking
Physics problem solving
Math problem solving
argumentation (related to critical thinking)
FAA Private Pilot and Instrument Pilot training material
where possible visits to towers, TRACON, etc.
volunteering at:  glider club, medvac company
public speaking
STEM college degree, not necessarily in aeronautical science

In college there will be more opportunities, and additional guidance.  Many high school students have learned to fly.  It's nice to have former students who are flying F-16s in various parts of the world.  Some who wanted to be fighter pilots ended up doing things like B-2s, tankers, etc.
Since most pilot jobs involve being an officer, leadership skills are important, and help differentiate applicants. You will always have to somehow differentiate yourself, to be competitive. Every former fighter pilot I know is very competitive.
Best.

One followup comment...a friend who was a military aviatior (he flew generals around) suggested several times that Military History was important to understand.  I agree, but it is not in my top list.  For example, understanding how American love of cars influenced the battlefield in Europe, in that yankee willingness (and permission) to twiddle with vehicles, tanks and engines, meant that American tanks were not sitting ducks waiting the Herr Mechanic when the tank broke down.  Others have suggested that Chinese war strategy books are good. They may be correct, I just have a different opinion

I’d start with something similar to the USAF’s Pilot Aptitude Test or the RAF’s COMPASS test to first evaluate your ability to be a pilot.  If you like, pursue a pilot’s license in Finland or do glider training.  I would not go out and begin some military pilot training regiment recommended here as it’s probably wrong and will misguide you.  If you are accepted to undergo flight training in the Finnish Air Force, they will teach you all you need to know there.
Some basic recommendations:

Get good grades - fighter pilot is s competitive job title.
Pursue extra curricular activities - aviation related, if possible.
Network with military pilots - find good people who will write letters of recommendation for you at some point
Pursue leadership training and opportunities- military pilots are officers, which is the military equivalent of a manager in civilian life.
I also suggest doing team sports like football or similar to build hand eye coordination and working on a team.

Best of luck.

You may want to study basic aviation, but I advise you to avoid anything military or fighter pilot specific.
All armed forces spend substantial effort "untraining" recruits who picked up bad habits and mis-information before entering the service.
Military service involves absolute consistency and uniformity.  Anything you learn on your own that deviates from the service's SOP will have to be unlearned before you can make progress.

You can't go wrong by picking up a copy of "Aerodynamics for Naval Aviators" -- see for example https://www.asa2fly.com/Aerodynamics-for-Naval-Aviators-P193.aspx --
It's certainly a few decades old by now, but full of excellent information.
Lots of interesting information about swept-wing flight dynamics and more--


text:
Do helicopters need a class B entry clearance when air / hover taxiing?
You're a helicopter pilot at a class B airport. You'd like to taxi to another parking spot. The ground controller could either give you a hover taxi clearance or an air taxi clearance, depending on the situation. Either way will get you off the ground and (technically) gets you to enter the class B airspace. Would you need to obtain a class B entry clearance prior to requesting your taxi clearance?

No. Hover taxiing with a helicopter follows the same regulations as taxiing with an aircraft (or helicopter) on wheels.

Does the same go for air taxiing?

Hover taxi cannot happen unless you are in the air....

What do you mean? I believe the commenter is talking about the regulations, not necessarily that you're taxiing on wheels. @JuanJimenez

@Daan, do you percieve that there is a difference between hover taxi and air taxi?  Because I am pretty sure they are the same...

Negative, at least not in the United States. If you're hover taxiing you're staying above the taxiways no higher than 25 ft AGL. If you're air taxiing you can fly anywhere within the airport boundaries at an altitude no higher than 100 ft AGL as long as you remain clear of traffic.  Reference: https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/helicopter_flying_handbook/media/hfh_ch09.pdf page 7 @MichaelHall

And hence, to hover taxi you must be in the air, not on wheels.

Thanks for clarifying the difference.  Still, in either case you are already in their space, and receiving clearance from the controlling authority who understands what is entailed in your request is sufficient.  That's a far cry different from someone on the outside asking to transit through the Bravo.

You need to be cleared into Class B airspace.  Are you suggesting that a helicopter on the ground at a Class B airspace FBO is not in Class B airspace?

No, but so are airplanes. Are you suggesting that even a GA airplane that just wants to reposition on the ramp needs a clearance into the class B?

Technically, even with wheels on the ground anything above the surface is in class B airspace if you are at the primary airport.  I have never been a helicopter PIC in class B so I cannot answer definitively, but if there is no intent to fly and you are cleared to airtaxi by ATC, (albeit a ground controller) that should be sufficient.
If you are actually facing this situation it wouldn't hurt to call on a landline ahead of time to clarify expectations with your local controllers.  In any case you wouldn't need to make a separate radio call for class B entry.  If you are truly concerned, this could be accomplished all at once.  For example:  "(callsign) request airtaxi in Bravo airspace from _ to _."  Then you have it on tape.

This was just hypothetical. Thanks for your answer.

This doesn't make sense to me. Taxi permission is dependent on the airfield. If it's controlled then you get permission from the tower ground controller, if it's uncontrolled it's pilot's discretion

@GdT Yes, I agree.  Nothing I said contradicts this.  What specifically doesn't make sense?


text:
Would it be possible to start a modern airliner from cold & dark during flight?
Imagine a suicidal pilot or disturbed passenger who gets access to cockpit, messes with the buttons. He shuts down engines, IRS, and battery, it's the middle of the night at cruise altitude.
Would it be possible to restart everything during flight and land safely ?

This is a pretty broad question, since it would depend on a lot of things. Aircraft type, altitude not to mention the human factors involved. Unless you are just looking for an answer saying "Yes, probably", you should try to narrow down your question and add more details.

if he vandalises the buttons, how could they then be used to restart the aircraft systems?

@jwenting I meant by vandalize just messing with state of buttons.

I'd not classify that as vandalism. Any competent pilot would not assume the state of anything when entering a clean cockpit, let alone one where an unauthorised person had gained entry.


text:
Is "11/2SM" a typo in this TAF message?
In a TAF-message downloaded from NOAA I came across "11/2SM" as the reported visibillity. I have previously seen "1 1/2SM" which I've been told to read as 1.5 SM.

is "11/2SM" a typo (should it have been "1 1/2SM") ?
Should it be read as a fraction "11/2" = "5.5" ?
Should it be read as "11+1/2" = 11.5" ?

Edit: Entire TAF (look 4th last line):
CYYZ 090839Z 0909/1012 31015G25KT P6SM FEW020 BKN050
TEMPO 0909/0913 P6SM -SHSN BKN020 BKN050
FM091300 29018G30KT P6SM -SHSN BKN025 OVC080
TEMPO 0913/0920 4SM -SHSN BKN020 OVC080
PROB30 0915/0920 11/2SM -SHSN
FM092000 31020G30KT P6SM -SHSN BKN025 BKN080
FM100100 33020G32KT 6SM -SN BKN020
FM101000 33018G28KT P6SM BKN020 RMK NXT FCST BY 091200Z

It's probably a typo, but it would help to read the entire TAF in context.

According to NOAA, there should be a space between the whole and fractional parts.  But they would never use a fractional part greater than 1, so your example must be a typo and should have read "1 1/2SM".


text:
When to turn during missed approach when MAP not identifyable
Not a duplicate of When to go missed on a non-precision approach? please read carefully before duplicating this contains some subtle points. Reposting after clarifications that emphasise this is a different question
According to AIM 5.4.21 (b): When an early missed approach is executed [...] fly the IAP as specified on the approach plate to the MAP at or above the MDA or DH before executing a turning maneuver.
Hence if a pilot decides on a missed approach, he needs wait until over the MAP to execute any turns of that procedure. One can only assume that is meant to prevent a climbing turn into a nearby protected airspace.
The following scenario puzzles me.
A pilot executes a LOC approach with a missed approach consisting of "Climb to 1000' then left climbing turn to 5000' direct to..." Sometime after the FAF the cockpit timer breaks. The MAP can no longer be identified by time. The pilot immediately starts climbing to 1000'. But when to start the left climbing turn to 5000'?

The aircraft does not have other equipment that would positively identify the MAP, when is that pilot required to begin that turn?
The aircraft has a DME and a functioning timer? Which is preferred to identify the MAP? Is the pilot allowed to change from timer to DME after having initially started using the timer for MAP identification upon timer becoming unusable or does equipment malfunction mandate execution of a missed approach?

This is a dupe of this question, however this one may have more useful answers.


text:
Is there ICAO document that describes MVA criteria?
According Skybrary article the minimum vectoring altitude in each sector provides 1000 ft above the highest obstruction in non-mountainous areas and 2000 ft above the highest obstacle in designated mountainous areas. Could someone advise ICAO or EUROCONTROL manual / document that contains definition of minimum vectoring altitude (MVA)? Which manual should I use to find information about 1000 ft above the highest obstruction in non-mountainous areas and 2000 ft above the highest obstacle in designated mountainous areas as MVA criteria?


text:
CG limits of Piper Pawnee?
In looking at a Piper Pawnee I found in the cockpit most of the expected documents, including a POH and a signed-by-A&P weight and balance sheet for that particular aircraft. The POH lists the max gross as 2900 lb, but I couldn't find anything documenting CG limits. Now, to be fair, this is a single seat, over the wing, it has a small baggage shelf behind the pilot's head, and the 38 gallon fuel tank. The agricultural spray hopper is removed, so  it's really hard to imagine that any possible loading would push it out of CG limit. That said, it seems to me that it's not entirely inconceivable that an FAA inspector might demand that I prove I'm in limits.
Does anyone have any information, ideally with a credible source, on what the limits are? Or, perhaps, an idea of where I could find such information? I suppose perhaps it's part of a type certificate, but I don't know how I'd find a copy of that?
Thanks for any suggestions/info!

The W&B report and POH should have all the data you need. It's pretty strange for the W&B sheet not to have the CG limits included.

Maybe they should, and I would agree that the POH suggests the W/B sheet for the airplane ought to have in this case, but for this airplane they don't (unless, perhaps, it's really well hidden in the POH). See page 18 in this (which is not the exact M/M I'm looking at, but this appears to be extremely similar POH, just a bit newer: https://www.soarccsc.com/wp-content/uploads/2015/09/CCSC-Pawnee-D-POH.pdf

The POH just refers to the sheet.  And there you are.  If the data isn't on the airplane's W&B report, you have a problem.  I tow gliders in a Pawnee and I've never checked it lol.

Yeah, embarrassing, isn't it. I've been flying Pawnees for 20 years, and this is the first time I've noticed. And that because locals are whining about noise, trying to close the airport, and complaining to the FAA that we're "not doing things right", so we're expecting, and preparing for, ramp checks.

Wow. If your sheet has the empty weights, moments and CG, maybe write to Piper for the W&B load chart?

Well, that's a thought. I just hoped maybe someone here might actually know the limits, but that's not looking promising so far.


text:
There is any technical reason for the new Airbus shades?
There is any technical reason for the new Airbus shades? Or they exists just because it is sexy?

I voted to reopen because the other question is about glass tint, and (I think) this one is about the frames.

It's not just for the looks. The A350 is the first Airbus ever to feature curved cocpit windows to reduce drag, and according to Airbus, the black "mask" contributes to ease of maintenance and harmonization of the thermal condition of the windows:

"The perfectly curved shape of the nose helps the air flow hug the surface, in the least turbulent manner, thereby reducing drag. The emblematic “Ray-Ban” like black windshield eases the window’s maintenance and contributes to harmonising the thermal condition of this temperature-sensitive window area. ..."

Virgin Atlantic blog - Why does the Airbus A350 wear a Zorro mask?

I notice Airbus a321 NEO also has that feature. Does it still have the same purpose as there isn't any noticeable design changes?

Purely speculative, but I think the black mask on NEO is just a brand gimmick to differentiate it from older versions. To be honest, I think the mask is mostly for cosmetic purposes on A350 too..


text:
Are there reference speeds like V1, Vr or V2 for a touch and go procedure?
Are there any reference speeds like V1, Vr and V2 on a touch and go procedure?
During takeoff, these are used to mark certain states of the takeoff, including the speed at which the aircraft starts pitching up (Vr -> rotate speed).
But during touch and go, are those predetermined values as well?

While for touch and go there isn’t a speed that carries the same guarantees as the takeoff speeds (you will not hit anything nor will you exit the runway on ground at undue speeds), for procedure reasons there are indeed predefined speeds at which to rotate and at which to climb. On the 777, for example, rotation is called at VREF (standard manual approach speed for selected flap setting) on touch and go, and climbout is flown at VREF+15 to VREF+25. .
In any case, the touch and go manoeuvre is only a training manoeuvre and is not in any way considered in performance calculations. Since it’s not calculated, it is also not a manoeuvre for which sufficient runway distance and/or obstacle clearance is assured by either calculation or regulation, neither on all engines nor following an engine failure, other than by flight crews’ experience and prudence. The actual speeds for touch and go are therefore somewhat more arbitrary than on takeoff.
The closest manoeuvres that are considered in performance calculations are a normal takeoff and a go-around from minimums. The relevant speeds for the former you know, for the latter it is the predetermined approach speed.


text:
Is an ekranoplan less efficient than a regular plane?
This are my conclusions upon researching the matter:
Ground effect is the cushion of air that can not escape down because of the ground. The hovercraft creates that cushion using a skirt which prevents as mush as possible a lateral escape of the air from underneath the ship. In effect a pressure differential between underneath and over the ship is created, enough to counter the ship weight. The value of this overpressure is within millibars.
Now returning to ekranoplan. While in the water, the ship has to fight the water drag. If the wings provided just enough lift to defeat the water drag, and the air cushion former under is rendered slow enough at escaping, then the ship becomes airborne. Will ride on a dynamically forming air cushion, better known as ground effect. However, the air drag becomes now the main opposing force. Increasing the angle of attack will increase the air drag and thicken the air cushion to the point where the ground effect is lost and the aircraft will loose altitude down to ground effect. Since the power output is limited, the machine is restrained at flying in ground effect only.
Now in terms of efficiency, the answer is no. The ekranoplan uses more thrust to fly in ground effect because it has to overcome air drag which is significantly stronger due to high angle of attack required to maintain the ground effect. The main reason the airplanes are efficient is due to low drag cruising. The wing develops enough lift to maintain the aircraft in the air while the angle of attack makes the wing a blade cutting through the air. Since the ekranoplan rides at way lower speeds than a regular airliner, it is required a high angle of attack which in turn determines a high drag.

This question looks like a dup of Can ekranoplans (GEV) be more efficient than traditional airliners?

There are a couple of inaccurate trains of thought in the question.

The ekranoplan uses more thrust to fly in ground effect because it has to overcome air drag which is significantly stronger due to high angle of attack required to maintain the ground effect.

All aeroplanes need to overcome air drag, which grows quadratically with airspeed. Ground effect creates more lift at a given speed, therefore requires less thrust at that speed. Or: ground effect creates more lift at a given wing area, requiring less thrust than the larger wing required for the conventional aeroplane.

...thicken the air cushion to the point where the ground effect is lost and the aircraft will loose altitude down to ground effect.

Thickening an air cushion cannot result in losing ground effect. The ekranoplane cannot fly out of ground effect - increasing Angle of Attack does not make much sense and altitude will auto stabilise to within ground effect, indeed at higher thrust level at higher AoA for a given speed. I believe this is what OP means, but it does not equate to ekranoplanes being less efficient than conventional aeroplanes. Ground effect creates extra lift without a drag penalty.
Whether ekranoplanes are more or less fuel efficient than standard aeroplanes is not due to the ground effect, but to the interference with the water surface, sea state, having to lift the engine out of the spray etc. Depending on the speed, they may be less efficient than ships, but the payload arrives a lot quicker.

The main problem with them I see is everybody is traveling at high speed at the same "flight level" of 20 feet or whatever.  Imagine a fleet of them traveling this way and that on the Black Sea trying to avoid running into each other.

@JohnK: Well, we have already figured out how to have millions of vehicles travelling at flight level 0 feet (mostly) without running into each other.

Roads and traffic lights.  Imagine a lake with dozens of really fast boats going this way and that.

We can see the increased efficiency of the design in this photo of the Orlyonok:

This is an aircraft roughly the size and carrying capacity of a midsize transport like a C130, yet its wings are much shorter, and it is propelled in flight by a single turbofan. Granted, probably more powerful than the C130's engines (looks similar to, and possibly the same unit used on the TU95), but one engine as compared to four.
This doesn't quantify the relative efficiency exactly, but the benefits are clearly visible.
However, ekranoplans require a flat surface to operate, so they are restricted to over water flight. What makes the ekranoplan impractical for general ocean use is the sea state requirement: no more than 3 foot swells. Higher than that, ground effects and lift are disturbed, and a wave striking the hull would stall the aircraft and result in a crash - just too low to recover from a stall. An ekranoplan, operating on the open ocean, would be in serious trouble if the sea state worsens while it is in flight, as sea states have been known to do. Even in calmer seas, one rogue wave could crash an ekranoplan.
In that respect, the ekranoplan is less efficient than a conventional aircraft, for the narrow range of weather conditions that it can operate. Much of the efficiency of modern transport aircraft is reliability of delivery: their ability to operate in almost any weather condition.
This was a primary reason the Soviet Navy lost interest in the ekranoplan technology: it's likely NATO opponents would not restrict operations to locales and weather conditions where an ekranoplan could reliably operate.
Note that Russia has shown some interest in reviving the design recently for commercial applications, and a few recreational aircraft based on the ekranoplan design have appeared in recent years, for use on lakes.

Doesn't it also have two jet engines (in the front), in addition to the turbofan on the tail?

@dbkk it does. I read somewhere there role was to increase ground effect for lift off (exhaust directed just below the front part of wings) and that the main engine generated the majority of thrust while in flight.

I'm curious about sea state requirements. Do you have any further readings?

there is a YouTube show dedicated to the development of ekranoplanes only. According to them, these ships were unstable, unable to turn or operate in waves beyound 6feet. The engines were affected by salty spray. The ground effect cushion was demanding for pilots. The only advantage was that on a calm day these ships were way faster than regular shipS, but only in straight line.

@WindSoul I would prefer a pdf with references i can check for further reading explaining how they get there results (with a description of there experimental methods)

We’re talking a military project developed in the ussr. The only reference I have provided is a public one. If you want further references I would recommend you first watch tube show, then try to contact the producer. That would be a sensible course of action.


text:
Does the FAA require the airlines to maintain continual communication with aircraft that can't be switched off?
Has it occurred to the FAA that all aircraft should be in constant communication with the network and report status periodically?
I'm thinking of the Malaysian Airways disappearance, where a jet flew towards the South Pole for 7 hours before it crashed. The pilot turned off the transponder and all communications. Only the low level hardware communication pings allowed us to know it was even alive.
Every 10 seconds, the aircraft should report very basic parameters like position, altitude, speed, heading, engine power, and fuel. You can put all that into a single IP packet.
Surely this must have occurred to someone? Are the airlines resistin
I'm talking about a coninuous once-per-10-second ping of a single packet of hardware status data to a central tracking database. My question had nothing at all to do  with anything happening in the cockpit, including the cockpit voice recorder.

Are you familiar with ADS-B, specifically Space-Based ADS-B?

This question is similar to: Live streaming cockpit to air traffic controllers. If you believe it’s different, please [edit] the question, make it clear how it’s different and/or how the answers on that question are not helpful for your problem. -- and see also this question referenced in my answer there. Everything electrical on an aircraft MUST have the capability to be deactivated.

It's amazing how the question "Does the FAA require the airlines to maintain continual communication with aircraft that can't be switched off?" can be associated with "Why don't all commercial aircraft transmit GPS data in real time?". Actually constantly flagging questions as duplicates of ones which are not but are somehow similar, prevents anyone to ask or answer with a bird's-eye view of any topic. Voting to reopen for the principle, but I know it won't be supported.

All evidence points to somebody in the MH370 cockpit having intentionally sabotaged every communication device aboard, including killing all power except what's provided by the ram air turbine because that's the only way to fully turn off satcom. Extra communication requirements or equipment would likely not have helped in a scenario where the pilot is intentionally trying to go dark and has planned it all out ahead of time.

@mins, how is this not a dupe of the others?

Yes, this is a really important question even if there is a snag with duplicates.  I am also voting to re-open because the summary by mins is concise and saves time by linking an overview of similar (or nearly identical) questions.

Does the FAA require the airlines to maintain continual communication with aircraft that can't be switched off?

No, there is no such requirement for radios, they can be switched off, they can fail, and propagation conditions may not allow to establish a radio link using terrestrial networks.

Every 10 seconds, the aircraft should report very basic parameters like position, altitude, speed, heading, engine power, and fuel. You can put all that into a single IP packet. Surely this must have occurred to someone? Are the airlines resisting

At the time of MH370 it was already possible using satcom and ACARS. ACARS can report a status using VHF or satcom. Satcom is used when overflying areas without VHF ground stations (desert, ocean).

MH370 had a satcom equipement to use ACARS, but didn't pay for the subscription, so they were not able to transmit ACARS when remote of VHF stations.

Why the subscription wasn't paid is likely because this would increase the ticket price, something passengers don't want, so airlines are indeed reluctant.

Nevertheless, considering the huge cost of Search and Rescue operations for MH370 and AF447, States are asking for the generalization of a space ATC. In space ATC communications with ATC can be done by multiple means, including satellites.

Space ATC is currently under preparation, but some elements are already deployed, e.g. the position can be reported from nearly everywhere using space-based ADS-B:

Aircraft equipped with ADS-B can stream their exact position and altitude twice per second, wherever they are in the world, to a network of 66 satellites.

Source: Space-based ADS-B helps NAV CANADA reach new safety milestone.
If we are able to follow aircraft overflying oceans and desert areas on flight tracking sites, this is because they continuously report their position using a satellite and space ADS-B.

This question is recurrent under multiple forms, focusing on different aspects, so this overview is already refined in:

Why don't all commercial aircraft transmit GPS data in real time?

What are the recent developments in the satellite-based tracking of aircraft?

Post MH370, are there developments to improve the tracking of airliners? (closed as duplicate)

Live streaming cockpit to air traffic controllers (closed as duplicate)

Would better location tracking for airliners improve safety?

Could the CVR and FDR record to the cloud?

The answers may be outdated though, as this is a field which is under active development.

"No, there is no such requirement for radios." It's abundantly clear from the context given in the body of the question that the OP is referring to data vs voice communication.

@MichaelHall But data is also transmitted over radio.

@void_ptr, I understand, but I specified data vs voice.  The question was asking about transmitting data, not technical details about the RF means of accomplishing that.  Anyway, if you believe this question is significantly different than the duplicates it was closed for then please edit to make that clear and I will gladly vote to reopen.

@MichaelHall: I'm answering "Does the FAA require the airlines to maintain continual communication with aircraft that can't be switched off?" the answer is no. There are specific cases to use some airspace classes where voice and transponder are required, but both radios can be turned off, and anyway I'm not detailing as the OP linked their question to MH370 example which was under oceanic control, not FIR or UIR (and not FAA, but that's a detail).

Okay, but it how is this different from the other questions and answers?

@MichaelHall: The criterion for duplicates, is not "this has been answered somehow elsewhere", "but this question and this other question are a single one". The difference between "is radio contact uninterruptible by regulation" and "Why GNSS position isn't transmitted in realtime" should be obvious. The fact the current question leads to mentioning spaced-based ACARS/ADS-B already detailed elsewhere as a solution to the underlying problem plays no role in the duplicate story. I'm perfectly able to re-route the OP to other related questions after answering the first specific question.

That's a pretty subtle and almost insignificant difference in my opinion.  You yourself stated "This question is recurrent under multiple forms, focusing on different aspects, so this overview is already refined in:" and then provided links to no less than a half dozen very similarly themed questions!  Anyway, I've made my point, DV for redundancy.

@MichaelHall: Anyway "duplicates" are not so bad for the different forms: Dr. Strangedupe: Or, How I Learned to Stop Worrying And Love Duplication: "bear in mind that cultivating and supporting a moderate amount of natural duplication actively helps the community", that's from Atwood himself.


text:
What are this sensor especially those at tips?
This is an image of an RQ-7 B UAV what are those sensors marked in red circles.

The tall bump at the front is "tactical common data link" (from this video)
The wingtip antenna is a com relay antennae based on this article


text:
Which propeller configuration is the one that generates the least turbulent flow in a wind tunnel?
I looked at the post here but still have some questions Why are push-propellers so rare, yet they are still around?
This is along the same idea but the answers are not too helpful either. Pros and cons of a pull-push propeller configuration
I am sw engineer so maybe this is a dumb question (if you can link resources i can read that would be the tits) but suppose you had an engine prop setup in three possible ways.

push prop behind the plane
pull prop in front of the plane
push and pull prop in the back/front respectively

Now suppose your not trying to fly, but just generate as much thrust as possible in a closed environment (or move as much air as possible like an AC duct system or wind tunnel).  Is the third setup superior?  If so by how much?  Assume the same engine/props in all setups.
Assuming the third setup is superior at moving air, if you keep adding props in a closed environment like a wind tunnel at some point you must hit diminishing returns.  If you had 100 pull/push/pull/push/pull etc in a row for example, would that be even better than 1 set of pull/push motors/prop setups?  I would imagine at some point the flow would be so disturbed that it would actually lower the airspeed.
I am trying to build a wind tunnel and i want to create the most airspeed possible.  I currently have designed 4 pull/push motors/props setup throughout the system but I dont know WHY i did this, it just seemed to make sense.  There are 8 DC motors with 5" props in a 6" piping system that loops back on itself.
EDITTED FOR CLARITY
To clarify what i am asking, I want to create a wind tunnel with the highest airspeed i can generate.  I have designed 4 sets of push-pull props (placed back to back on electric motors) and each set is placed along this closed system (wind tunnel) in different places where i think the air would slow down.  In my head each set will resemble the coaxial rotors from this video:

I am looking for insight as to if this is worth doing, and how they scale.  Is a single prop better than having the two back to back forcing air in the same direction?  If there are gains by adding more props to the system, is there a limit where the flow just becomes so turbulent that it actually pushes air slower?
I could put 8 props all forward along the way, or i could put 4 sets of push-pull props, or i could use some number less than 8.  In this image i plan on placing the 4 pairs of motors/props where the arrows are:
I apologize for the confusing question, im a relative aviation noob compared to the people here.  I can try to re-ask this question if things are still confusing, but please help me ask a better question next time (if im using the wrong terminology or missed a tag or whatever)
Thanks in advance.

I have clarified the question, can you put my question off of hold or help me ask a better question?

You might want to adjust your YouTube link to start the video a couple of seconds before the part you're referencing. I've watched more than a minute of it and I don't think I've come to the relevant location yet.

@FreeMan that was a good idea, I have updated it to the relevant time.

Pusher props are less common than "puller" props for the following reasons.
A "puller" prop spins around in air which has not yet been disturbed by the passage of the airplane through it. This means the airflow over the prop blades will be as close to ideal as possible, and the amount of thrust the prop will generate will also be close to the theoretical maximum possible for that prop design.
A "pusher" prop spins around in air that has already been disturbed by the passage of the airplane's wings and fuselage through it: it is working in the wake of the airplane, and as it spins, it moves in and out of the wake of the wing or the fuselage. Each blade will generate a little less thrust in the disturbed air inside the wake and a little more thrust in the undisturbed air outside the wake.
This cyclic variation in thrust sets up vibration in the propeller shaft and radiates sound waves away from the prop. The plane is therefore noisier both inside the plane and outside.
In general, this means that a pusher prop will almost always be less efficient than a puller prop, and this will be true in a wind tunnel too because all the same principles apply.
The situation is actually worse in a wind tunnel, because you want to have the smoothest and least turbulent air flow possible in the tunnel so you can get the most accurate and consistent data out of it.
The worst combination in a wind tunnel would be a series of pusher/puller combinations because these would be adding the most turbulent, swirling air motions into the airstream.

How many percent will be the thrust different in the disturbed inside the wake and in the undisturbed outside the wake?

Ask peter kampf on this site, he knows more about this than I do... "Kampf" has an umlaut over the "a" which I do not know how to type- but he da MAN!

Thanks for the answer.  Would you say that having a pusher/puller combination is worse at generating airspeed than a single pusher or puller?  I dont think i care about smoothest flow as much as i care about fastest flow, perhaps that is misguided though.

@Peter Kämpf can you help my understanding with this idea?  Wont i get more airspeed with more props?

@peterkampf, can you please help us in this matter?

@AirCraft Lover I have added info to the question, does it help convey what i am asking?  I am looking to push air in a square like apparatus as with as high of airspeed as possible, and i would like to find a resource (or just info) on what the best configuration would be (or tradeoffs to consider as thats often the case).


text:
Why Engine Exhaust Nozzle can not be totally buried in aircraft fuselage?
Why The CD nozzles of the fighter aircraft are always outside the fuselage, Why the whole engine is not buried in the fuselage??
If engine shroud also can be made variable area and varies with CD nozzle will it be possible to bury the whole engine inside fuselage???

Can you think of any advantage to putting a shroud around the nozzle?

To reduce IR signature???

Some aircraft do what you're proposing, like the F-22 (photo below) and the F-117. So the technique has been known since the 1970s.

These are exceptions though. To effectively reduce the IR signature, you don't just need a shroud, you need to cool the exhaust stream. This means the design of the aft fuselage gets much more complicated, and it gets heavier (and if you use hydraulics to shape the shroud around the exhaust, that adds more weight).
In all designs, there are tradeoffs to be made. If a design feature isn't common, that usually indicates there are drawbacks, in this case the improvement in IR signature doesn't outweigh increased weight and complexity.

The nozzle in the F-22 is still at the exit. It’s a variable area 2D converging-diverging nozzle. The reason you do not put the nozzle inside the fuselage, is because then you need a duct after that. Within that duct, the static pressure isn’t equal to ambient pressure, because of the constraining effect of the duct. So, the gas isn’t expanding to ambient static pressure through the nozzle, and hence the nozzle’s performance is significantly reduced. The exit at the end of the duct will still act like a nozzle. If that’s a fixed area, it will not work well over a high speed range

With an afterburning engine you can't bury the engine forward and use a long duct to the exhaust nozzle because the length of the afterburner chamber is limited, and the afterburner nozzle has to be the last thing the supersonic air passes through.  If there is no afterburner, the exhaust duct can be fairly long with a simple convergent nozzle at the end (like an F-86, which has its engine midships with a long exhaust duct).
The nozzle on an afterburning engine does double duty.  Burner off, it's a normal taipipe with simple convergent nozzle tip.  Burner on, the nozzle morphs into a venturi shape (a convergent/divergent duct, like a rocket exhaust) to manage the supersonic flow created by the afterburner.  So the nozzle has to be at the back, and the engine has to be close to the nozzle, how close determined by the length of the afterburner chamber.
You could put an afterburning engine amidships, but you'd then have to have a high tail to clear the exhaust nozzle which now has to be ahead of the tail (like an F-101 Voodoo).  But if you want to also use the nozzles for steering, you want them as aft as possible to get the most pitch authority.  So the engines on the F-22 end up at the tail and the only airplanes you see with fully buried engines and longish shrouded exhaust ducts (F-117 and B-2) are subsonic a/c without afterburners.

Did you mean F-117 in the last sentence? F-111 Aardvark is a swing-wing low-penetration light bomber that has afterburner and is supersonic. F-117 Nighhawk, on the other hand, is the ugly edgy first stealth and is subsonic.

Yes.  Thanks Jan.

You can also have an engine amidships (lovely word), then a long jetpipe, and then the afterburner just at the tail. Like engine 1 (the lower one) in the English Electric / BAC Lightning.

Well there goes my thesis up on smoke LOL.  Is there an article somewhere that shows how they pulled that off?


text:
By using the speed brake do we reduce the speed or do we increase the descent rate or does both happen simultaneously?
By using the speed brake do we reduce the speed or do we increase the descent rate or does both happen simultaneously? Could someone please explain the concept

Related question: What is the relation between aircraft descent rate and speed?

It depends entirely on what else you do with pitch and power.  Because you could hold the same speed and altitude but just burn more fuel.

The two most common types of speed brakes are fuselage mounted and top of the wing mounted speed brakes.
The effect of both types is the increase of drag. Top of the wing mounted speed brakes also decrease lift.
The "end result" of using speed brakes is actually up to the pilot to decide.
First use case would be the "both" scenario: most efficient approach profile considering fuel consumption would be to simply idle glide from cruising altitude all the way to the final approach fix. Unfortunately traffic situations and geographical features do not always permit this, and therefore trajectory management may be necessary: the plane might need to descent at faster rate than would be possible for a given airspeed restriction. In this case the pilot would choose to use the speed brake to achieve both the steeper descent rate and airspeed.
Second use case would be simple speed management: modern passenger planes are quite "slippery": they can be hard to slow down. in these cases it might be necessary to use speed brakes to bleed off the airspeed to match the traffic flow and approach procedures of the airport. In this case, the speed brake are applied to slow down while maintaining "normal" descent rate or even level trajectory.
Top of wing mounted speed brakes / spoilers require some trim adjustment to maintain the trajectory of the plane. The speed brake itself may or may not induce this trim change. Fuselage mounted ones such as on BAe-146 are pretty much trim neutral.

Every implementation of a speed-brake that I've come across is basically a method of increasing drag, rather like putting your foot on the ground with a scooter, and slightly differently from most wheeled vehicles, which only brake indirectly.
Secondary effects on the aircraft are exactly that - 'secondary' - and entirely dependant both on what the aircraft is doing, and what controls are in play. For example, if the aircraft has auto-throttle, and the speed brake is somehow deployed without the management system being aware, then the throttles will be advanced until the aircraft regains the selected airspeed.

The practical answer is they increase angle of descent.
How they do it is a bit more complicated.  Aircraft in level flight at constant speed have the all familiar 4 forces of flight:  thrust and drag, lift and gravity, in balance.
The speed brake creates more drag, slowing the aircraft, which reduces lift.  The aircraft, if it is staticly stable, will pitch down until thrust + gravity component (sine angle of descent × weight) = drag
Since lift is proportional to velocity squared, aircraft pitch (to the horizon) will be very sensitive to a reduction in airspeed.
If the aircraft is not staticly stable (most are), then the reduction of speed will be more noticable.
Another case to consider is that the speed brake, such as a flap, can increase the coefficient of lift (of the wing).  In this case, the aircraft will have both a decrease in airspeed and an increase in angle of descent.


text:
Why there is a 'MACH' in this Compressibility Correction Chart?

I couldn't find what 'Mach 1.0' and 'Mach 0.8' are trying to indicate in the middle of the chart.
Can anybody explain about this please?

As I read it, the upper arch pointed to by Mach 1.0 represents the correction necessary at the different pressure altitudes when flying at Mach 1.0, while the lower arch pointed to by Mach 0.8 represent the correction necessary at those pressure altitudes when flying at Mach 0.8.
i.e. the correction necessary at 30,000 feet at Mach 1.0 is roughly -30, while the correction at that same altitude but Mach 0.8 is only roughly -15.

The M 1 curve is uncorrected calibrated airspeed at the speed of sound at various pressure altitudes and is the limit of the chart, and the M .8 curve is the uncorrected calibrated airspeed at M .8 at the various pressure altitudes, roughly (M# is actually a function of temperature; pressure alt is used as a substitute based on standard atmosphere temperature and lapse rate).
They are just there for reference.  If use of the chart required the precise plotting of Mach # as a parameter, there would be an array of Mach # plots, not just the one at .8.


text:
What is 'dual monitoring' in radio communication?
I heard this word while local flying, and can't find any other results on internet.
I only knew that 'monitoring a frequency' means only listenening to the selected frequency, rather than making a communication channel.
What is 'dual monitoring', and how is it different from 'monitoring a frequency'?

Further to the answer provided...  It's a common practice among some aviators to always be monitoring COM2, since it's there for free anyway.  Tuning it to the emergency freq, (121.50 in the US) and listening "just in case" is called being "on guard."  Or if you are VFR in an area where there's approach control, you can dial that up and listen in.  Also quite useful in IFR flight because you really need to stay on your controller's freq because you can go get ATIS, as the answer suggests.

In an aircraft with two or more comm radios, you can only transmit on one, but you can listen to more than one frequency.
So "dual monitoring" this would be listening (and transmitting) on one frequescy, while also listening on a second frequency. This is common when you're in contact with one ATC, but maybe you're passing close to perhaps a busy airport where you might want to monitor their movements. Another good example is monitoring an ATIS while also talking to an approach controller.
This is a pretty standard radio panel in most light aircraft

The knob to the right selects which comm radio you transmit on. The button with illumination marked COM1 is which comm radios you're monitoring. You might usually have just COM1 selected, but you can also select COM2 (and/or COM3) to dual monitor. As you can see from the above image it is possible to have 3 comm radios wired to this box.


text:
What is the difference between 'warning area' and 'ADIZ'? Are they overlapped with each other?
On FAR AIM, definition of warning area is

Airspace of defined dimensions,
  extending from three nautical miles outward from the
  coast of the U.S, that contains activity that may be
  hazardous to nonparticipating aircraft

And on Wikipedia, definition of ADIZ is

Airspace over land or water in which the identification, location, and control of civil aircraft is performed in the interest of national security.

I was confused because the definition of these two airspace is similar with each other. And I thought that those area could be overlapped with each other. Could anyone explain the difference, or at least an example chart that contains this two areas?
And if an aircraft tried to fly to the U.S territory from the sea, which area does the aircraft encounter first : 'warning area' or 'ADIZ'?

You can find a good explanation of Warning areas (and lots of other areas) in the answer to this question.
ADIZ is effectively the controlled area that surrounds the US/CANADA border. The picture from the wiki article shows it quite well

The ADIZ was created to have an area to identify inbound, out of country, aircraft. As per the FAA

All aircraft entering U.S. domestic airspace from points outside must
  provide for identification prior to entry or exit. ADIZs have been
  established to assist in early identification of aircraft in the
  vicinity of international U.S. airspace boundaries (AIM Section 6,
  5-6-1)

As far as I know there are no ADIZ areas internal to the country, although warning areas may be anywhere within the nations jurisdiction. I see no reason they could not overlap each other either.

To clarify your statement, warning areas exist outside US airspace (starting 3 NM off the coast.) They work similar to Restricted airspace, but because it isn't US airspace the US can't restrict air traffic. They just warn you that there is hazardous activity. There's a lot of air-to-air weapons testing in the Warning areas over the Gulf of Mexico near Eglin AFB, FL.

Is it correct to say that the ADIZ also exists along the US/Mexico land border but is infinitesimally thick?

@pericynthion  Not exactly. The ADIZ 'near side' is displayed on the charts as a line. The line is the point where you enter/exit US airspace. In coastal areas, the ADIZ extends out to the extent of the US FIR which is the area under US air traffic control. At the Mexican border the line follows the border, and the the other side is Mexican airspace. The US can't define/control any airspace over Mexico, but that doesn't mean they (both US and Mexico) aren't watching. And with a flight plan, you'll get a handoff from/to the Mexican ATC.